Schottky barrier rectifiers are used extensively as output rectifiers in switching-mode power supplies and in other high-speed power switching applications such as motor drives, for carrying large forward currents and supporting high reverse blocking voltages. As is well known to those having skill in the art, a two-terminal rectifier exhibits a very low resistance to current flow in a forward direction and a very high resistance to current flow in a reverse direction. As is also well known to those having skill in the art, a Schottky barrier rectifier produces unipolar rectification as a result of nonlinear current transport across a metal-semiconductor contact.
There are basically four distinct process for the transport of predominantly unipolar charge carriers across a metal/N-type semiconductor contact. The four processes are (1) transport of electrons from the semiconductor over a metal/semiconductor potential barrier and into the metal (thermionic emission), (2) quantum-mechanical tunneling (field emission) of electrons through the barrier, (3) recombination in the space-charge region and (4) hole injection from the metal to the semiconductor. In addition, edge leakage currents, caused by high electric fields at the metal contact periphery as well as interface currents, caused by the presence of traps at the metal-semiconductor interface, may also be present.
Current flow by means of thermionic emission (1) is the dominant process for a Schottky power rectifier with a moderately doped semiconductor region (e.g., Si with doping concentration .ltoreq.1.times.10.sup.16 /cm.sup.3), operated at moderate temperatures (e.g., 300K). Moderate doping of the semiconductor region also produces a relatively wide potential barrier between the metal and semiconductor regions and thereby limits the proportion of current caused by tunneling (2). Space-charge recombination current (3) is similar to that observed in a P-N junction diode and is significant only at very low forward current densities. Finally, current transport due to minority carrier injection (4) is significant only at large forward current densities.
As the voltages of modern power supplies continue to decrease in response to need for reduced power consumption and increased energy efficiency, it becomes more advantageous to decrease the on-state voltage drop across a power rectifier, while still maintaining high forward-biased density levels. As well known to those skilled in the art, the on-state voltage drop is dependent on the forward voltage drop across the metal/semiconductor junction and the series resistance of the semiconductor region and cathode contact.
The need for reduced power consumption also requires minimizing the reverse-biased leakage current. The reverse-biased leakage current is the current in the rectifier during reverse-biased blocking mode of operation. To sustain high reverse-biased blocking voltages and minimize reverse-biased leakage currents, the semiconductor portion of the rectifier is typically lightly doped and made relatively thick so that the reverse-biased electric field at the metal/semiconductor interface does not become excessive. The magnitude of the reverse-biased leakage current for a given reverse-biased voltage is also inversely dependent on the Schottky barrier height (potential barrier) between the metal and semiconductor regions. Accordingly, to achieve reduced power consumption, both the forward-biased voltage drop and reverse-biased leakage current should be minimized and the reverse blocking voltage should be maximized.
Unfortunately, there is a tradeoff between the forward-biased voltage drop and the reverse-biased leakage current in a Schottky barrier rectifier, so that it is difficult to minimize both characteristics simultaneously. In particular, as the Schottky barrier height is reduced, the forward voltage drop decreases but the reverse-biased leakage current increases. Conversely, as the barrier height is increased, the forward voltage drop increases but the leakage current decreases. The doping level in the semiconductor region also plays a significant role. The higher the doping level, the lower the forward-biased voltage drop but reverse-biased breakdown is reduced by impact-ionization.
Therefore, in designing Schottky barrier rectifiers, design parameters such as barrier heights and semiconductor doping levels are selected to meet the requirements of a particular application because all device parasitics cannot be simultaneously minimized. Low barrier heights are typically used for Schottky rectifiers intended for high current operation with large duty cycles, where the power losses during forward conduction are dominant. High barrier heights are typically used for Schottky rectifiers intended for applications with higher ambient temperatures or requiring high reverse blocking capability.
The height of the Schottky barrier formed by the metal/semiconductor junction is related to the work function potential difference between the metal contact and the semiconductor substrate. A graphical illustration of the relationship between metal work function and Schottky barrier height may be found in Chapter 5, FIG. 3 of the textbook by S. M. Sze entitled Semiconductor Devices, Physics and Technology, John Wiley & Sons, 1985, at page 163. A detailed and comprehensive discussion of the design of Schottky barrier power rectifiers may be found in Section 8.2 of the textbook entitled Modern Power Devices by coinventor Baliga, published by John Wiley and Sons, Inc., 1987, the disclosure of which is hereby incorporated herein by reference.
In particular, sections 8.2.1 and 8.2.2 of the Baliga textbook disclose the semiconductor physics associated with both forward conduction and reverse blocking in a parallel-plane Schottky rectifier having the structure of FIG. 8.13 of the Baliga textbook. As set forth in Equation 8.14, the forward voltage drop is dependent on the drift region, substrate and contact resistances (R.sub.D, R.sub.S and R.sub.C), as well as the forward current density (J.sub.F) and saturation current (J.sub.S), which is a function of the Schottky barrier height (.theta..sub.Bn). The maximum reverse blocking voltage (i.e., breakdown voltage) of a Schottky rectifier (BV.sub.pp) is also disclosed as ideally being equal to that of a one-sided abrupt P-N junction rectifier (e.g., P.sup.+ -N or N.sup.+ -P), having the structure of FIG. 3.3 of the Baliga textbook. The breakdown voltage BV.sub.pp is dependent on the doping concentration of the drift region (N.sub.D), as described by Equation (1) below. EQU N.sub.D =2.times.1O.sup.18 (BV.sub.pp).sup.-4/3 ( 1)
Equation (1) is a reproduction of Equation 8.18 from the aforementioned Baliga textbook. A graphical representation of breakdown voltage and depletion layer width (W.sub.pp) at breakdown versus drift region doping (N.sub.D) for an abrupt P-N junction rectifier is shown by FIG. 1. FIG. 1 is a reproduction of FIG. 3.4 from the aforementioned Baliga textbook.
In reality, however, the actual breakdown voltage of a Schottky rectifier is about one-third (1/3) that for the abrupt parallel-plane P-N junction described by Equation (1) and graphically illustrated by FIG. 1. As will be understood by those skilled in the art, the reduction in breakdown voltage below the parallel plane value is caused, in part, by image-force-induced lowering of the potential barrier between the metal and the semiconductor regions, which occurs at reverse-biased conditions.
One attempt to optimize the on-state voltage drop/reverse blocking voltage tradeoff associated with the Schottky barrier rectifier is the Junction Barrier controlled Schottky (JBS) rectifier. The JBS rectifier is a Schottky rectifier having an array of Schottky contacts at the face of a semiconductor substrate with corresponding semiconductor channel regions beneath the contacts. The JBS rectifier also includes a P-N junction grid interspersed between the Schottky contacts. This device is also referred to as a "pinch" rectifier, based on the operation of the P-N junction grid. The P-N junction grid is designed so that the depletion layers extending from the grid into the substrate will not pinch-off the channel regions to forward-biased currents, but will pinch-off the channel regions to reverse-biased leakage currents.
As will be understood by those skilled in the art, under reverse bias conditions, the depletion layers formed at the P-N junctions spread into the channel regions, beneath the Schottky barrier contacts. The dimensions of the grid and doping levels of the P-type regions are designed so that the depletion layers intersect under the array of Schottky contacts, when the reverse bias exceeds a few volts, and cause pinch-off. Pinch-off of the channels by the depletion layers causes the formation of a potential barrier in the substrate and further increases in the reverse-biased voltage are supported by the depletion layer, which then extends into the substrate, away from the Schottky barrier contacts. Accordingly, once a threshold reverse-biased voltage is achieved, the depletion layers shield the Schottky barrier contacts from further increases in reverse-biased voltage. This shielding effect prevents the lowering of the Schottky barrier potential at the interface between the metal contacts and semiconductor substrate and inhibits the formation of large reverse leakage currents.
The design and operation of the JBS rectifier is described in Section 8.4 of the above cited textbook and in U.S. Pat. No. 4,641,174 to coinventor Baliga, entitled Pinch Rectifier, the disclosures of which are hereby incorporated herein by reference. For example, as shown by FIG. 6 of the '174 patent, reproduced herein as FIG. 2, an embodiment of a pinch rectifier 200 comprises a plurality of Schottky rectifying contacts 232 formed by metal layer 230 and substrate 204 and a P-N junction grid formed by regions 234 and substrate 204. Unfortunately, the JBS rectifier typically possesses a relatively large series resistance and a relatively large forward voltage drop caused by the reduction in overall Schottky contact area dedicated to forward conduction. This reduction in area is necessarily caused by the presence of the P-N junction grid at the face of the substrate. In addition, large forward currents can cause large forward voltage drops and can lead to the onset of minority carrier conduction (i.e., bipolar), which limit the JBS's performance at high switching rates. Finally, although the reverse blocking voltage for the JBS may be somewhat higher than the reverse blocking voltage for a Schottky rectifier having an equivalent drift region doping (N.sub.D), it does not achieve the level of reverse blocking capability obtainable with a parallel-plane P-N junction, as shown by FIG. 1.
Another attempt to optimize the forward voltage drop/reverse blocking voltage tradeoff is disclosed in U.S. Pat. No. 4,982,260 to Chang, coinventor Baliga and Tong, entitled Power Rectifier with Trenches, the disclosure of which is hereby incorporated by reference. For example, as shown by FIGS. 10B and 14B, reproduced herein as FIGS. 3 and 4, respectively, conventional P-i-N rectifiers (P.sup.+ -N.sup.- -N.sup.+) are modified to include an interspersed array of Schottky contacts on a face of an N-type semiconductor substrate. As shown by FIG. 3, the Schottky contact regions 550A-C are separated from the P+ portions 510A-D (of the P-i-N rectifier) by MOS trench regions 522A-522F. In another embodiment shown by FIG. 4, the Schottky contact regions 718A-E are interspersed adjacent the P+ portions 720A-F, which are formed at the bottom of trenches 710A-F. As will be understood by those skilled in the art, these modified P-i-N rectifiers also typically possess an unnecessarily large series resistance in the drift region (N.sup.- -type regions 506, 706). Moreover, only a relatively small percentage of forward-conduction area is dedicated to the Schottky contacts, which dominate the forward bias characteristics by turning on at lower forward voltages than the parallel connected P.sup.+ -N junctions. Finally, although the forward leakage current for these P-i-N type rectifiers is substantially lower than corresponding forward leakage current for a Schottky rectifier, like the JBS, they do not achieve the level of reverse blocking capability associated with an abrupt parallel-plane P-N junction.
AMOS barrier Schottky (MBS) rectifier has also been proposed to allow for unipolar conduction at forward voltage drops greater than 0.5 Volts and at forward unipolar current densities greater than those obtainable using the JBS rectifier, described above. In particular, this MBS rectifier was described in an article entitled New Concepts in Rectifiers, Proceedings of the Third International Workshop on the Physics of Semiconductor Devices, Nov. 27-Dec. 2, World Scientific Publications, Madras, India, 1985, by coinventor Baliga. As shown by the performance simulation curves of FIG. 5 of that article, ideal reverse blocking voltages on the order of 150-1200 volts represent the theoretical limit for an MBS rectifier having a relatively high forward voltage drop greater than about 0.5 volts. The simulation curves of FIG. 5 were based on Equations 8.14 and 8.18 of the aforementioned Baliga textbook. Accordingly, the curves of FIG. 5 assume ideal Schottky rectifier behavior and, as noted above, do not take into account effects such as image-force-induced lowering of the potential barrier between the metal and the drift region.
Thus, notwithstanding these developments, there continues to be a need for a Schottky rectifier having low forward voltage drop and high reverse blocking capability, and preferably a Schottky rectifier having an ideal or near ideal parallel-plane blocking voltage.